The electric power steering apparatus for assist-load energizing the steering apparatus of a car or a vehicle with rotation force of a motor is designed to assist-load energize a steering shaft or a rack shaft with a driving force of the motor by a transmission mechanism such as a gear or a belt through a speed reducer. In such a conventional electric power steering apparatus, feedback control of motor current is performed to precisely generate assist torque (steering assist torque). The feedback control is for adjusting voltage to be applied to the motor such as to reduce a difference between a current command value and a motor current detection value, and the voltage to be applied to the motor is generally conducted by adjusting a duty ratio of a PWM (pulse width modulation) control.
A general structure of the electric power steering apparatus will be explained with reference to FIG. 7. A column shaft 2 of a steering wheel 1 is connected to a tie rod 6 of a running wheel through a speed reducing gear 3, universal joints 4a and 4b and a pinion rack mechanism 5. The column shaft 2 is provided with a torque sensor 10 for detecting the steering torque of the steering wheel 1. A motor 20 for assisting the steering torque of the steering wheel 1 is connected to the column shaft 2 through the speed reducing gear 3. Electricity is supplied to a control unit 30 which controls the power steering apparatus from the battery 14 through an ignition key 11 and a relay 13. The control unit 30 calculates a steering assist command value I of assist command based on the steering torque T detected by the torque sensor 10 and based on vehicle speed V detected by the vehicle speed sensor 12, and the control unit 30 controls current to be supplied to the motor 20 based on the calculated steering assist command value I.
The control unit 30 mainly comprises a CPU (including MPU also), and general functions of the control unit 30 executed by program in the CPU are shown in FIG. 8. For example, a phase compensator 31 is not a phase compensator as independent hardware, but is a phase compensating function executed by the CPU.
The functions and operation of the control unit 30 will be explained. The steering torque T which is detected and inputted by the torque sensor 10 is compensated in phase by the phase compensator 31 for enhancing the safety of the steering system. The steering torque TA which was compensated in phase is input to a steering assist command value calculating element 32. The vehicle speed V detected by the vehicle speed sensor 12 is also input to the steering assist command value calculating element 32. The steering assist command value calculating element 32 determines the steering assist command value I which is a control target value of current to be supplied to the motor 20 based on the input steering torque TA and vehicle speed V. The steering assist command value I is input to a subtracter 30A and is also input to a differentiation compensator 34 of a feedforward system for enhancing the response speed. A deviation (I-i) of the subtracter 30A is input to a proportion calculating element 35 and is also input to an integration calculating element 36 for improving the characteristics of the feedback system. The output of the differentiation compensator 34 and outputs of the proportion calculating element 35 and the integration calculating element 36 are added and input to an adder 30B. A current control value E which is a result of addition in the adder 30B is input to a motor drive circuit 37 as a motor driving signal. The current i of the motor 20 is detected by a motor current detection circuit 38 and is fed back to the subtracter 30A.
In such a conventional electric power steering apparatus, in order to make full use of the flexibility in design thereof and to obtain stable and comfortable steering feeling, it is proposed to design the transmitting characteristics of the steering feeling and the road information sensitivity in the frequency region (e.g., Japanese Patent Application Laid-open No. 2001-334948). That is, the complementary sensitivity function with respect to frequency of the control apparatus is set a value approaching “1” in a band where there exists disturbance to be suppressed, and is set to a value approaching zero in a band where there exists disturbance to be transmitted. This is because that under the definition of the complementary sensitivity function, when the function is “1”, this means that the disturbance is suppressed, and when the function is zero, this means that the disturbance is not suppressed at all and is transmitted.
It is conceived that the steering feeling and the road information sensitivity can be used as indices of performance evaluation in the electric power steering apparatus. However, the conventional electric power steering apparatus has a problem that since the transmitting characteristics representing these two indices have mutual subordinative relation, it is difficult to design a control system which satisfies the two indices at the same time. That is, since the steering feeling and the road information sensitivity have the mutual subordinative relation, the steering feeling and the road information sensitivity can not be designed independently, and it is difficult to design a control apparatus which satisfies the ideal steering feeling and road information sensitivity at the same time.
The present invention has been accomplished in view of the above circumferences, and it is an object of the invention to provide an electric power steering apparatus comprising a control system with two degree of freedom which is capable of independently designing the steering feeling and road information sensitivity for realizing ideal steering feeling and road information sensitivity at the same time in a frequency region.